The direction of extension, with respect to the orientation of the layering, imposes a strong control on the geometry of the boudins.

Extension parallel to layering led to symmetric boudinage. Thinning and necking in the stronger layers often initiate via the development of extensional fractures orthogonal to the layering. As extension proceeds, boudins evolve with a symmetric shape, and boudins do not rotate during extension.

Left: Example of extensional fractures orthogonal to bedding in a nascent neck region, the photo is ca. 8 cm long (Photo credit: P. Rey).
Right: Example of symmetric boudinage of a granitic vein in a psammitic schist (Broken Hill, NSW, Photo credit: P. Rey).